Phased arrays create beamed radiation patterns in free space to allow the formation of selective communication channels. A phased array is formed by placing a plurality of antennas in a grid pattern on a planar surface where these antennas are typically spaced ½ of the wavelength of the radio frequency (RF) signal from one another. The phased array can generate radiation patterns in preferred directions by adjusting the phase and amplitude of the RF signals being applied to each of the antennas. The emitted wireless RF signals can be reinforced in particular directions and suppressed in other directions due to these adjustments. The wireless beam is steered electronically to send a communication channel, thereby eliminating the need to adjust the position or direction of the antennas mechanically.
A phased array requires the orchestration of the plurality of antennas forming the array to perform in unison. A corporate feed network provides the timing to the phased array by delivering identical copies of an RF signal to each of the plurality of antennas forming the phased array. A uniform placement of the plurality of antennas over a planar area defines the phased array as having a surface area that extends over several wavelengths of the carrier frequency of the RF signal in both of the X and Y directions. For example, a phased array with 100 antennas arranged in a square planar area would have edge dimension equal to 5 wavelengths of the RF carrier frequency in each direction.
In cellular transmission, orthogonal frequency-division multiplexing (OFDM) is adopted by modern systems such as Long-Term Evolution (LTE) due to its resistance to intersymbol interference (ISI) and low-complexity in channel equalization. However, OFDM signals typically exhibit a high peak-to-average power ratio (PAPR). To maintain linearity of the transmitted radio frequency (RF) signal, high PAPR requires large power amplifier (PA) back-off (i.e., increasing the PA supply voltage to increase the source power of the supply relative to average transmitted RF signal power). The increased supply power provided to the PA insures that the PA is operating linearly when the RF signal is at its peak level. When the RF signal returns to its average level, the PA suffers a high power loss which is not converted to RF signal power. The additional power loss translates into increased heat dissipation within the PA. Thus by increasing the PA back-off to improve the linearity of the PA, it also causes a reduction in the PA efficiency and increased power usage. In many cases, limits on the heat dissipation or power consumption of the PA can become the bottleneck on the performance of the entire radio transmission system.
This situation becomes more severe in phased arrays since there is a plurality of PAs, each one of the plurality of PAs adding its heat output, due to the PA driving one of the antennas in the phased array. All of the PAs can be placed near the antennas, the antennas defining the planar area of the phased array. The close placement of the PAs together generates a significant amount of heat in a relatively small volume associated with the phased array. The heat dissipation becomes more severe if the PAs experience a high PAPR issue as mentioned earlier. The increased power loss of the PAs due to a high PAPR can generate a significant amount of heat in a very small volume. Moreover, a phased array may process multiple beam signals, and at the input of each PA, these beam signals are typically rotated and added. Techniques are required to reduce the high PAPR that PAs experience when multiple beam signals are added together. Reducing the PAPR allows the PA to become more power efficient.